Simple Dark Sensor Circuit with Buzzer

A simple dark sensor circuit with a buzzer is an easy to make electronic project that can detect when there is no light and then make a sound.

This kind of circuit can be used in different areas like automatic night lights, security systems and alarms.

It works by using a LDR along with a 555 timer chip and a buzzer which drives to produce an alarm sound.

When the light around it gets too low the circuit turns on the buzzer to let people know that it is dark.

Also, it can be powered by a 9V PP3 battery, which makes it handy and easy to use for many different purposes.

Circuit Working:

Parts List:

The Simple Dark Sensor Circuit with Buzzer uses an IC 555 timer set up in a bistable mode.

In this arrangement a LDR works together with resistor R1 to form a voltage divider that controls the reset pin 4 of the integrated circuit 555 .

When there is light the LDR has low resistance, which means the voltage at the reset pin is high preventing the circuit from turning on.

But when it gets dark the LDRs resistance increases causing the voltage at the reset pin to fall below a certain level.

This drop in voltage turns on the output, making the buzzer sound an alarm.

VR1 pot helps protect the buzzer by controlling how much current comes from pin 3 of the 555 timer.

R3, which is a LDR has a low resistance when it is bright outside (about 1kΩ to 10kΩ) and a high resistance when it is dark (around 100kΩ to 1MΩ).

This change in resistance affects the voltage at trigger pin 2, which activates the circuit when it gets dark.

The capacitor C1 is important because it filters out any unwanted noise making sure the 555 timer does not get triggered by random voltage changes.

This keeps the circuit running smoothly.

Formulas with Calculations:

The formulas with calculations for Simple Dark Sensor Circuit with Buzzer are mentioned below:

Voltage Divider Formula:

The voltage across the LDR is determined by the voltage divider rule:

V_LDR = V_CC × (R2 / (R2 + R3))

where,

• V_LDR is the voltage across the LDR
• V_CC is the supply voltage of 9V in the circuit
• R2 is the fixed resistor 1M
• R3 is the LDR resistance varies with light intensity

Trigger Voltage of IC 555 Timer:

The 555 timer triggers when the voltage at pin 2 falls below 1/3 of the supply voltage:

V_trigger = V_CC / 3

For a 9V power supply:

V_trigger = 9V / 3 = 3V

LDR Resistance Values:

The resistance of the LDR changes based on light conditions:

LDR resistance in light = 1kΩ to 10kΩ

LDR resistance in darkness = 100kΩ to 1MΩ

Voltage Across LDR in Light and Dark Conditions:

In bright light (assuming LDR = 10kΩ):

V_LDR_light = 9V × (1MΩ / (1MΩ + 10kΩ))

V_LDR_light = 8.91V

Since this voltage is much higher than 3V the 555 timer remains off.

In darkness (assuming LDR = 500kΩ):

V_LDR_dark = 9V × (1MΩ / (1MΩ + 500kΩ))

V_LDR_dark = 6V

Since this voltage is still above 3V the circuit does not activate yet.

For activation (assuming LDR = 1MΩ in total darkness):

V_LDR_dark = 9V × (1MΩ / (1MΩ + 1MΩ))

V_LDR_dark = 4.5V

If we need the circuit to activate at a higher light level, we can reduce the resistor R2 to adjust the trigger threshold accordingly.

How to Build:

To build a Simple Dark Sensor Circuit with Buzzer following steps are required to be followed for connections of the circuit:

• Gather all the components mentioned in the above circuit diagram
• Connect pin 1 of IC1 555 to GND of the circuit
• Connect pin 2 to pin 6 of IC1
• Connect capacitor C1 from pin 2 of IC1 to GND
• Connect resistor R2 one end from pin 2 of IC1 and other end of R2 resistor connect to pin 3 of IC1.
• Connect pin 3 of IC1 to one end of VR1 pot and other end of pot connect to one end of 9V buzzer and other end of buzzer connect to GND
• Connect pin 4 and pin 8 of IC1 to positive supply of +9V battery.
• Connect resistor R1 between pin 4 and pin 8 of IC1.
• Connect one end of LDR R3 from positive supply of battery and other end of LDR connect to GND

Conclusion:

This Simple Dark Sensor Circuit with Buzzer is an easy and efficient method to sense darkness and send out a warning.

You can change the resistor values to adjust how sensitive it is based on what you need.

This circuit can be used for security alarms, night lights or even as an automatic light switch to save energy.

References:

Block diagram of LDR sensor.